The pharmacokinetics of the adenosine deaminase inhibitor, 2'-deoxycoformycin (dCF) in the plasma, as well as the level of cyclic AMP, adenosine and deoxyadenosine in the urine of patients undergoing Phase I and II clinical trials is currently being assessed. The mechanism of action of the immunosuppressive effect of dCF is being determined in vitro with T and B lymphocyte cultures from mouse spleen and thymus. Measurements of adenosine metabolites, and DNA, RNA and protein synthesis will be correlated with the suppression of blastogenesis by dCF resulting from the mitogenic response of the cells to concanavalin A and lipopolysaccharide.